Noise removing apparatus in an FM receiver

Abstract

A noise removing apparatus in an FM receiver includes a high-pass filter (29) receiving an output from an FM detector circuit (11), a gate (15) being interrupted in response to a pulsive noise detected by level-discriminating an output from a variable gain amplifier (35) receiving an output from the high-pass filter (29). Transistors are provided for controlling the gain of the variable gain amplifier (35) according to the magnitude of a terminal voltage of a capacitor which is charged according to an output from a rectifying circuit (43). The noise removing apparatus further includes a voltage generator (51) for generating a voltage proportional to a received electric field strength and a circuit (53) for varying an amount to be charged in the capacitor according to the magnitude of a voltage from the voltage generator (51) for maintaining the gain of the variable gain amplifier (35) large at the time of strong electric field, so that a multipath noise can be effectively removed.

What is claimed is: 
     
       1. A noise reducing apparatus in an FM receiver comprising: means for receiving a frequency modulated signal and generating an intermediate frequency signal therefrom;   means coupled to said FM receiving means for FM detecting said intermediate signal and generating an output signal therefrom, said output signal including noise signals comprising pulsive noise and continuous noise resulting from multipath noise;   output means for receiving said output signal;   gate means coupled between said detecting means and said output means for interrupting the receipt of said output signal by said output means;   pulsive noise detecting means coupled to said FM detecting means for detecting at least said pulsive noise, said pulsive noise detecting means comprising variable gain amplifier means for amplifying substantially only said noise signals and providing a signal representative thereof, and automatic gain control means adapted to receive said amplified noise signals for controlling the gain of said variable gain amplifier means in response to said continuous noise included in said noise signals so that at least said pulsive noise is detected;   gate control means for opening said gate means in response to said amplified noise representative signal thereby interrupting receipt of said pulsive noise by said output means and removing at least said pulsive noise;   electric field strength detecting means for detecting an electric field strength of said FM signal and for providing an output signal representative thereof; and   automatic gain control effect controlling means responsive to said electric field strength detecting means output signal for controlling the gain control effect of said variable gain amplifier means resulting from said automatic gain control means, said multipath noise being detected by said pulsive noise detecting means when said electric field strength detecting means detects an electric field having a field strength greater than a predetermined value and said pulsive noise detecting means further provides an output signal for enabling said gate means to interupt the receipt of said multipath noise by said output means.   
     
     
       2. A noise removing apparatus in accordance with claim 1, wherein said automatic gain control means comprises automatic gain control voltage generating means (41) for generating an automatic gain control voltage corresponding to an amplitude of an output of said variable gain amplifying means (35), and   said automatic gain control effect controlling means comprises automatic gain control voltage reducing means (53) for reducing an automatic gain control voltage generated from said automatic gain control voltage generating means (41) according to an output from the said electric field detecting means (9, 51).   
     
     
       3. A noise removing apparatus in accordance with claim 2, wherein said automatic gain control means comprises rectifying means (43) for rectifying an output from said variable gain amplifying means (35),   amplitude detecting means (45) for amplitude detecting an output of said rectifying means (43), and   a capacitor (471) being charged according to an output of said amplitude detecting means (45), a terminal voltage thereof being applied as a control voltage to said variable gain amplifying means (35), and     said automatic gain control voltage reducing means comprises current reducing means (434, 452, 531) responsive to an output of said electric field strength detecting means (9, 51) for reducing a charging current flowing into said capacitor (471).   
     
     
       4. A noise removing apparatus in accordance with claim 3, wherein said current reducing means comprises means (434) for limiting a rectifying gain of said rectifying means (43). 
     
     
       5. A noise removing apparatus in accordance with claim 3, wherein said current reducing means comprises means (452) for limiting a detecting gain of said amplitude detecting means (45). 
     
     
       6. A noise removing apparatus in accordance with claim 1, wherein said gate control means comprises a monostable multivibrator (33),   said automatic gain control means comprises automatic gain control voltage generating means (55) for generating an automatic gain control voltage corresponding to the duration time period of an output of said monostable multivibrator (33),   said automatic gain control effect controlling means comprises automatic gain control voltage reducing means (53) for reducing an automatic gain control voltage generated from said automatic gain control voltage generating means (55) according to said electric field strength detecting means (9, 51).   
     
     
       7. A noise removing apparatus in accordance with claim 6, wherein said automatic gain control means comprises a capacitor (551) being charged during the duration time period of an output from said monostable multivibrator (33), the terminal voltage thereof being applied as a control voltage to said variable gain amplifying means (35), and   said automatic gain control voltage reducing means comprises current reducing means (531) responsive to an output from said electric field strength detecting means (9, 51) for reducing a charging current flowing into said capacitor (551).   
     
     
       8. A noise removing apparatus in accordance with claim 2 or claim 7, which further comprises resistors (472, 552) connected in a parallel manner to said capacitors (471, 551), and wherein said automatic gain control voltage reducing means comprises means (531) substantially connected in parallel to said resistors (472, 552) and responsive to an output of said electric field strength detecting means (9, 51) for bypassing a current flowing into said resistors (472, 552).   
     
     
       9. A noise removing apparatus in accordance with any of claims 1, 2-5 wherein; said receiving means comprises an intermediate frequency circuit including an amplifying means for amplifying said intermediate frequency signal and a limiter means for limiting an output amplitude from said amplifying means; and   said electric field strength detecting means comprises means for generating a voltage proportional to said electric field strength of said FM signal corresponding to a magnitude of an output signal received from said intermediate signal amplifying means.   
     
     
       10. A noise reducing apparatus in an FM receiver comprising: means for receiving a frequency modulated signal and generating an intermediate frequency signal therefrom;   means coupled to said FM receiving means for FM detecting said intermediate signal and generating an output signal therefrom said output signal including noise signals comprising pulsive noise and continuous noise resulting from multipath noise;   output means for receiving said output signal;   gate means coupled between said detecting means and said output means for interrupting the receipt of said output signal by said output means;   pulsive noise detecting means, coupled to said FM detecting means for detecting at least said pulsive noise, said pulsive noise detecting means comprising high pass filter means coupled to said detecting means for passing substantially only said noise signals to an output terminal thereof, variable gain amplifier means coupled to said high pass filter output terminal for amplifying said noise signals, noise signal detecting means for comparing said amplified noise signals to a predetermined level and providing a signal representative thereof, and automatic gain control means adapted to receive said amplified noise signals for controlling the gain of said variable gain amplifier means in response to said continuous noise included in said noise signals so that at least said pulsive noise is detected;   gate control means for opening said gate means in response to said representative compared signal for removing at least said pulsive noise;   electric field strength detecting means for detecting an electric field strength of said FM signal and for providing an output signal representative thereof;   automatic gain control effect controlling means responsive to said electric field strength detecting means output signal for controlling the gain control effect of said variable gain amplifier means resulting from said automatic gain control means, said multipath noise being detected by said pulsive noise detecting means when said electric field strength detecting means detects an electric field having a field strength greater than a predetermined value, said pulsive noise detecting means further providing an output signal for enabling said gate means to interrupt the receipt of said multipath noise by said output means.